1. Field of the Invention
This invention relates to integrated circuit (IC) fabrication methods, and particularly to methods of integrating microelectromechanical system (MEMS) devices with low-resistivity silicon substrates.
2. Description of the Related Art
Microelectromechanical system (MEMS) devices such as MEMS switches have found a wide applicability, due to their very low loss and low power characteristics, as well as their ability to operate at RF frequencies. These characteristics make MEMS devices ideal for use in portable communications equipment. For example, MEMS switches and/or variable capacitors are well-suited for use in digital tunable filters having low-loss and high-Q factors.
Experience with present-day MEMS devices indicates that they perform best when built on a substrate material having a very high resistivity ( greater than 10,000 xcexa9-cm), as substrates having a lower resistivity tend to degrade RF performance. Unfortunately, high resistivity substrates are often not compatible with circuitry which is interconnected to the MEMS devices. For example, the silicon-germanium (SiGe) BiCMOS fabrication process enables major portions of an RF transceiver to be built on a common substrate. However, SiGe is a low-resistivity substrate material, which makes integration with MEMS devices difficult or impractical. This typically results in the SiGe circuitry and the MEMS devices being fabricated independently on separate substrates, which are then packaged together in a hybrid structure. While functional, hybrid packages are typically larger, more expensive, and less reliable than ICs.
Two methods of integrating MEMS devices with other, non-MEMS circuitry are presented. The methods overcome the problems noted above, providing smaller, cheaper and more reliable devices than were previously possible.
One method, referred to as xe2x80x9cdirect integrationxe2x80x9d, constructs the MEMS device(s) and the non-MEMS circuitry on a common substrate. Non-MEMS components are fabricated on a substrate in a conventional fashion. A thick dielectric layer, preferably polyimide, is deposited on the completed components, and the MEMS device(s) fabricated on the dielectric layer. The dielectric layer provides the high resistivity needed for superior RF MEMS performance, while the layer""s thickness reduces parasitic capacitance and enables low-loss transmission lines to be fabricated. Vias through the dielectric layer interconnect the MEMS devices to the non-MEMS electronics. The presence of the interposed dielectric layer allows the common substrate to have the characteristics that best suit the non-MEMS components, without degrading the MEMS performance.
Another approach, referred to as xe2x80x9cwafer level interconnectxe2x80x9d, involves bonding together two separate wafersxe2x80x94one for the MEMS device(s) and one for the non-MEMS electronics. A package lid is prepared which will encapsulate the MEMS devices; vias are formed through the lid and backfilled with a conductive material. The lid is bonded to the MEMS wafer, and hermetically seals all the MEMS devices on the wafer. The wafer holding the non-MEMS electronics is mounted to the side of the lid opposite the MEMS wafer, with the vias effecting the necessary interconnections between the two wafers. This enables the MEMS devices and the other electronics to function as a single IC, while retaining the established processes associated with each component type.
Further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.